Tilt mirror microelectromechanical system (MEMS) devices are used in a number of optical applications involving steering optical beams. Most commonly, they are used as beam switches. They route optical signals between various optical fibers. In this application, they can be used in low port count switches such as two-by-two (2xc3x972) switches to systems having very large fabrics with hundreds to thousands of ports.
In the most typical configuration, each tilt mirror comprises a mirror body on which a reflective coating has been deposited. Torsion arms extend from this mirror body to a support. Electrostatic forces are used to tilt the mirror body relative to the support on the torsion arms by establishing electrostatic fields between the mirror body and fixed electrodes.
A more complex example of a tilt mirror system is a tip/tilt mirror. Torsion arms extend between the mirror body and a support along one axis and then, another set of torsion arms extend between the support and a final outer support along an orthogonal axis. In this way, the mirror can be rotated around two orthogonal axis or in two dimensions.
The cores of single mode optical fiber, which is common throughout the communications industry, are small, between five and ten micrometers in diameter. As a result, when coupling optical signals between optical fibers, the tilt mirrors must be positioned to precise angular positions.
In the first implementations of the MEMS tilt mirror arrays, the angular position of the tilt mirrors was controlled simply by varying the electrostatic voltages. During the manufacture, a map of electrostatic drive voltage to angular position can be generated. During operation, a desired angular position is then achieved by reference to the corresponding electrostatic drive voltage in a look-up table.
These systems, without positional feedback, had a number of problems, however. They were sensitive to drift, orientation, and vibration, thus requiring periodic recalibration of the look-up tables.
More recently, position detection systems have been integrated with the MEMS tilt mirror systems. Specifically, capacitive sensors have been deployed. In these systems, the instantaneous position of the tilt mirror can be determined by reference to the capacitance between the tilt mirror and a fixed electrode. This enables finer control of the tilt mirror position, makes the arrays less susceptible to vibration, and makes them robust against acceleration due to gravity, for example.
One problem associated with these capacitive sensors, however, is long-term drift. The capacitance changes that the sensor must detect are small. Consequently, changes in the temperature of the electronics and aging result in apparent changes in mirror position. As a result, even with capacitive sensing, these tilt mirror arrays must still be calibrated. The typical approach is to guide a calibration beam to a fixed detector. This provides a known reference against which the position detection systems can be calibrated.
For each mirror, a fixed source/detector pair can be employed, which can contribute to the cost of the final module. Many times in these systems, a stopped mirrors are proposed when the mirrors must simply move between two different states. Physical stops are provided within the mirror""s range of movement and the mirror is simply driven into engagement with the stops.
The drawback with stopped mirror systems, however, concerns the fact that the manufacturing tolerances must be controlled so that different mirrors have the same angular orientation when engaging these stops. Mirror to mirror precisions of less then 0.1 degrees is typically required.
The present invention is directed to a movable MEMS mirror system with a mirror position detection system such as a capacitive sensor. The position detection system, however, can be calibrated without a calibration signal generator and detector. Specifically, stops are provided and position detection system for the mirror structure is then self-calibrated in the field relative to these stops.
In general, according to one aspect, the invention features a movable MEMS mirror system. It comprises a movable mirror structure having a range of movement. A mirror actuation system is used to move the mirror structure. Further, a stop is provided. A mirror structure position detection system then electrically monitors a position of the moveable mirror structure within the range of movement. A calibration system calibrates the mirror structure position detection system in response to a detected position of the moveable mirror based on contact with the stop. As a result, drift in the mirror structure position detection system can be compensated.
Typically, the movable mirror structure comprises a reflective metal coating such as aluminum or gold. In other cases, a dielectric thin film mirror coating is used.
The mirror structure typically comprises a mirror body and at least two hinges or torsion arms that connect the mirror body to a support. In still other embodiments, this support is then connected to a further outer support by two additional torsion arms, for example, to enable tip-tilt movement of the mirror structure.
In some embodiments, the stop is positioned over the mirror structure, whereas in other implementations, the stop is located under the mirror structure. In one implementation, the stop is positioned within a range of movement of the mirror structure. Thus, the moveable mirror structure can be moved into direct contact with the stop.
In more detail, the calibration system controls the mirror actuation system to pivot the mirror structure into contact with the stop and then calibrates the mirror structure position detection system in response to this known reference point.
In another embodiment, a moveable calibration structure is provided, along with a calibration structure position detection system, which monitors the position of the calibration structure. In this example, the calibration structure contacts the mirror structure and the stop. The calibration system then determines a position of the mirror structure in response to the distance the calibration structure moves between contact with the stop and contact with the mirror structure. The system has advantages in that the mirror structure does not need to be moved to perform the calibration process.
In general, according to another aspect, the invention features a method for calibrating a position detection system of a MEMS mirror system. This method comprises actuating a movable mirror structure in response to positional information from a mirror structure position detection system. To compensate for drift in this detection system, periodically, calibration of the mirror structure position detection system is performed based on the position of a stop.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.